Simulating Dynamic Bandwidth
Allocation on Satellite Links
Alberto Gotta, Francesco Potortì, Raffaello Secchi
Italian National Research Council (CNR)
Istituto di Scienza e Tecnologie dell’Informazione “A. Faedo” (ISTI)
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Summary
• DVB-RCS overview
• Architecture of Tdma-dama
– Extension to NS2
– Demand Allocation Multiple Access
– Rate Based allocation policy
• Validation & Measurements
• Conclusions
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
DVB-RCS system
• DVB-RCS (ETSI EN 301 790)
RETURN LINK
DVB-RCS
FORWARD LINK
DVB-S
– Bidirectional channels over satellite
links (Meshed Networks)
– It defines an MF-TDMA medium
access control
– It guarantees IP/DVB connectivity
via satellite links
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
DVB-RCS network elements
Forward
Link
Return
Link
TT
RNCC
RNCC
NCC
Gateway
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Tdma-dama objectives
• Deployment of DVB-RCS features into ns-2
– Integration of TDMA access scheme into the ns-2
satellite environment
– Development of bandwidth allocation methods
(CRA, RBDC, VBDC, FCA)
• Study of the interaction between transport and
satellite MAC layer
– Congestion control (TCP, TCP friendly) over satellite
links with dynamic bandwidth allocation
– Impact of MAC delay over delay-critical applications
(e.g VoIP, video-conferencing, etc.)
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Tdma-dama in ns-2
• The current satellite module of ns-2 supports
both geostationary satellite and polar orbiting
LEO constellations with multiple users per
downlink/uplink and asymmetric links
• Tdma-dama is a module in the MAC layer block
which supports dynamic bandwidth allocation
mechanisms
– IP packets fragmentation and encapsulation into a
TDMA frame
– Fully customizable from Otcl Interface
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
DAMA allocation principle
• Each traffic terminal periodically submits to a master
terminal an allocation request based on the incoming
traffic rate or traffic backlog at the terminal
Traffic
Terminals
(Requesters)
request
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Master
(Allocator)
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
DAMA allocation principle
• The master station collects the request of all the active
traffic terminals, computes a burst time plan and
broadcasts it back to the terminals
Traffic
Terminals
(Requesters)
Pisa –October 10th 2006
allocation
Master
(Allocator)
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Drawbacks of Dynamic Allocation
• The time interval between the TTs requesting a given
bandwidth and the system assigning it is called
allocation delay
• Due to the large allocation delay on the satellite link,
we can observe that the transmission delay
dramatically increases when the rate of traffic
incoming to a TT varies rapidly
– if the input rate increases in a time shorter than the allocation
delay, the excess traffic is buffered at the traffic terminal, thus
increasing the queuing delay
• Our simulative analysis confirms experimental findings
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Bandwidth Allocation Methods
The following methods can be combined in many ways:
• Constant Rate Assignment (CRA)
– Bandwidth is negotiated between the traffic terminal and the
allocator at the beginning of each connection
• Rate Based Dynamic Capacity (RBDC)
– Each Traffic terminals submits to the allocator a bandwidth
request message based on the rate of local incoming traffic
• Volume Based Dynamic Capacity (VBDC)
– Each terminal dynamically signals the data volume needed to
empty its buffer
• Free Capacity Assignment (FCA)
– No explicit request comes from the terminals. Unused bandwidth
is assigned automatically by the allocator to the traffic terminals
according to some fairness criteria
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Tdma-dama Design
• Tdma-dama consists of two customizable
C++ classes of agents
– The Requester agent, one for each satellite
terminal, is responsible for calculating the
bandwidth needed by the terminal and for
submitting a request to the allocator.
– The Allocator agent computes the burst time
plan, that is, the TDMA frame allocation for all
stations.
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Diagram of Tdma-dama
Traffic
terminal
Terrestrial
network
MAC
buffer
Requester
agent
TDMA
fragmentation
and reassembly
Pisa –October 10th 2006
PHY Tx/Rx
Raffaello Secchi – WNS2
Satellite
network
Allocator
agent
satellite
link
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Otcl Interface
main parameters
•
•
The MAC/TDMA block is created
through the procedure node-config
consistently with the standard ns-2
procedure and configured through a
set of class parameters
Variable name
Meaning
max_slot_num_
slots per frame
slot_packet_len_
size of a slot
num_frame_
allocation period
bandwidth_
satellite link capacity
A single system-wide Allocator is installed in a terminal:
set alc [$ter install-allocator Allocator/Proportional]
•
One Requester is installed into each terminal:
set req [$ter install-requester Requester/RDBC]
•
The definition of Requester and Allocator must specify the subclass of
the objects. The object name can be used at run time to dynamically
change the properties of Allocator and Requester
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Example: RBDC Requester
• A generic rate- and volume-based request:
q(t )
Rreq (t )  R(t ) 
Ts
where R(t) is the mean incoming traffic rate and q(t) is the
incoming queue length at the requesting terminal
• This expression can be interpreted as a proportional-integral
controller of output rate (q(t) being the integral part), which
improves the stability of the queue
• Stability can be further improved by providing a minimum
share to each station in addition to the requested bandwidth
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Test-bed platform for validation
DVB-RCS
DVB-S
LAN
Ethernet
LAN
Ethernet
testbed specifications
channel capacity
active stations
available bandwidth
2.1 Mb/s
Medium Access Control
TDMA/TDM
•
The bandwidth allocation uses a
DAMA scheme
•
It implements a variant of Rate
Based Dynamic Capacity (RBDC)
18
~1.2 Mb/s
slots per frame
48
frame duration
273 ms
bandwidth granularity
44 kb/s
guaranteed bandwidth
44 kb/s
mean RTT
840 ms
Pisa –October 10th 2006
•
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Throughput Comparison (ON/OFF traffic)
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Delay Comparison (ON/OFF traffic)
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Conclusions
• We described Tdma-dama, a simulator of TDMA satellite MAC
based on ns-2 that allows investigation of a large class of DAMA
schemes, including those outlined in the DVB-RCS standard
• We validated the simulator through measurements performed on
the Skyplex Data network, which implements a variant of the
RBDC allocation mechanism
– The statistics considered were throughput and one-way delay
– The simulator was able to accurately reproduce the dynamics of the
real system
• The simulations highlight that DAMA algorithms are responsible
for the large delay (several times the propagation delay) that may
occur in response to abrupt variations of input rate
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
How to get the software?
The software is provided under a MIT free software license
http://votos.isti.cnr.it/simulator.htm
Pisa –October 10th 2006
Raffaello Secchi – WNS2
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”